All people study or work for a business using a desk or a table in an office and conference room of a school, academy institute or public organs or various industrial fields, sitting on chair. In this case, almost chairs have backrests for safely supporting a user's back.
Generally, since a chair backrest supports a back of a user sitting on a chair, the user can take a rest in comfortable posture. In addition, the user's back can be protected in such a manner that a user's back is properly supported. When sitting on a chair, it is needed to sit on a chair in an upright posture in such a manner that a hip part of a user is closely contacted with a rear side of a seat, and a user's back is contacted with a backrest.
Therefore, the backrest of a chair is basically designed to safely protect a user's back based on a human body mechanic system.
The conventional chair has a structure that a backrest is elastically supported in a backward direction, and a dual-back structure that a backrest is divided into two parts for thereby effectively supporting both sides of a backbone of a user.
In the conventional chair, since a backrest of a chair is fixed at a set position irrespective of a position that a user sits, a user must move his hip to a deep corner of a seat in order to sit in upright posture and a user's back must be supported by a backrest, so that a user can sit on a chair in an upright posture. In the case that a user sits on a chair in a manner that a hip part of a user is positioned at an intermediate portion of a seat or is positioned at a front side of a seat, the backside of a user cannot be properly supported by a backrest of a chair. If the user works for a long time in a state that the user did not sit on a chair in an upright posture, a user's back may receive over load, so that a user may feel fatigue within a short time period. In addition, an abnormal phenomenon may occur in a backbone. Namely, the user may have a certain disease such as a back disc, etc.
A student studying on a chair or a worker working on a chair may sit at a rear side of a seat with his hip portion being positioned at a rear side of a sear, without sitting on a chair in an upright posture. If the user works in the above posture for a long time, since the user's upper body is not safely supported by a backrest of a chair, the user may feel fatigue faster. In more serious case, an over load may be applied to a user's back, so that a certain disease such as a back disc may occur in a user's back.